Traditionally, combustible gas detectors used to detect the presence of combustible gases such as those found in coal mines or manufacturing facilities, utilized a circuit configuration comprised of at least one sensing element. This sensing element was a wire having a catalytic coating. The sensing element was used as one of four legs of a wheatstone bridge circuit. The other three legs consisted of two resistors and a compensator element. The compensator element was identical to the sensing element except that it did not bear a catalytic coating.
A current or voltage was applied to the bridge circuit to heat the surface of the catalytic coating affixed to the sensing element. Since the resistance values of the other three legs of the bridge were known, the resistance in the sensing element could be determined as the current or voltage was passed through the bridge.
When the sensing element was exposed to a combustible gas, such as methane, the catalytic coating would begin to burn increasing the temperature of the sensing element. As the temperature of the sensing element increased, the resistance of the element increased. Accordingly, the current or voltage passing through the element decreased. By comparing the resistance level of the sensing element to the resistance level of the compensator element, the presence of a combustible gas could be detected. Since the amount of gas present caused a nearly linear increase or decrease in the resistance of the sensing element, the quantity of the gas could be accurately determined by calibrating the change in resistance. This is the basic principal of operation of a catalytic combustible gas sensor.
The actual use of this configuration does, however, possess drawbacks. For example, in the presence of high concentrations of combustible gases, the temperature rise of the catalytic coating can reach temperatures high enough to damage the sensing element. One method of alleviating this problem is to regulate the voltage or current that is passed through the sensing element. As heating causes the voltage or current to rise, a regulating circuit commonly known in the art, can be attached to the bridge to draw off some of the current or voltage. By reducing the current or voltage, harmful temperature extremes can be curtailed. The introduction of a regulating circuit can, however, introduce other errors that affect sensor performance. For example, regulating the voltage to the entire bridge circuit has relatively little effect on the sensor temperature. Also, another common practice, adjusting the bridge voltage to maintain a stable sensor voltage, is somewhat more effective, but does not stabilize the sensor sufficiently to prevent thermal damage.
Several combustible gas detectors that incorporate bridge circuits are known in the art. For example, Ferraro, U.S. Pat. No. 4,652,831, discloses a catalytic combustible gas detector that provides increased sensitivity to gases by pre-adjusting the voltage applied to the terminals of the sensing element. In this arrangement, the voltage must be pre-adjusted to the desired level dependent upon the type of gas to be tested. A second disclosure, Meyer E.P.C. Patent No. 018,221 provides an apparatus that incorporates two independent bridge circuits, one for the sensing element and one for the compensator element. The use of two circuits, however, requires twice the power consumption of a single bridge sensor. This can critically limit the portability of the instrument when the circuit is driven by batteries.
It is the intention of this invention to provide a combustible gas detecting circuit configuration that stabilizes the temperature of the sensing element to eliminate outside interference signals and thereby enable the circuit to provide faster response times, increased sensitivity and more accurate analyses of a gas in a manner that requires low power consumption while employing a single bridge circuit.